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A tunnel is an underground passageway. The
definition of what constitutes a tunnel is not universally agreed
upon. However, in general tunnels are at least twice as long as
they are wide. In addition, they should be completely enclosed on
all sides, save for the openings at each end. Some civic planners
define a tunnel as in length or longer, while anything shorter than
this should be called an underpass or a chute. For example, the underpass beneath Yahata Station in Kitakyushu, Japan is only long
and therefore should not be considered a tunnel.

The central part of a rapid transit network is usually built in
tunnels. To allow non-level crossings, some lines run in deeper
tunnels than others. Rail stations with much traffic usually
provide pedestrian tunnels from one platform to another, though others use
bridges.

Geotechnical investigation

It is essential that any tunnel project starts with a comprehensive
investigation of ground conditions. The results of the
investigation will allow proper choice of machinery and methods for
excavation and ground support, and will reduce the risk of
encountering unforeseen ground conditions. In the early stages, the
horizontal and vertical alignment will be optimized to make use of
the best ground and water conditions.

In some cases, conventional desk and site studies will not produce
sufficient information to assess, for example, the blocky nature of
rocks, the exact location of fault zones, or stand-up times of
softer ground. This may be a particular concern in large diameter
tunnels. To overcome these problems, a pilot tunnel, or drift, may
be driven ahead of the main drive. This smaller diameter tunnel
will be easier to support when unexpected conditions occur, and
will be incorporated in the final tunnel. Alternatively, horizontal
boreholes may sometimes be used ahead of the advancing tunnel
face.

Construction

Tunnels are dug in various types of materials, from soft clay to
hard rock, and the method of excavation depends on the ground
conditions.

Cut-and-cover

Cut-and-cover is a simple method of construction
for shallow tunnels where a trench is
excavated and roofed over. A strong overhead
support system is required to carry the load of the covering
material, roads, streets or other transportation systems.

Two basic forms of cut-and-cover tunnelling are available:

Bottom-up method: A trench is excavated, with ground
support as necessary, and the tunnel is constructed within. The
tunnel may be of in situ concrete, precast concrete, precast
arches, corrugated steel arches and such, with brickwork used in
early days. The trench is then backfilled, with precautions
regarding balancing compaction of the backfill material, and the
surface is reinstated.

Top-down method: In this method, side support walls
and capping beams are constructed from ground level, using slurry
walling, contiguous bored piles, or some other method. A shallow
excavation is then made to allow the tunnel roof to be constructed
using precast beams or in situ concrete. The surface is then
reinstated except for access openings. This allows early
reinstatement of roadways, services and other surface features.
Excavation machinery is then lowered into the access openings, and
the main excavation is carried out under the permanent tunnel roof,
followed by constructing the base slab.

Shallow tunnels are often of the cut-and-cover type (if under
water, of the immersed-tube type), while deep tunnels are
excavated, often using a tunnelling
shield. For intermediate levels, both methods are
possible.

Large
cut-and-cover boxes are often used for underground metro stations,
such as Canary Wharf
tube station in London. This construction form generally
has two levels, which allows economical arrangements for ticket
hall, station platforms, passenger access and emergency egress,
ventilation and smoke control, staff rooms, and equipment rooms.
The interior of Canary Wharf station has been likened to an
underground cathedral, owing to the sheer size of the excavation.
This contrasts with most traditional stations on London Underground, where bored tunnels
were used for stations and passenger access.

Boring machines

Tunnel boring machines (TBMs) and associated back-up systems can be
used to highly automate the entire tunneling process. There are a
variety of TBMs that can operate in a variety of conditions, from
hard rock to soft water-bearing ground. Some types of TBMs,
bentonite slurry and earth-pressure balance machines, have
pressurised compartments at the front end, allowing them to be used
in difficult conditions below the water
table. This pressurizes the ground ahead of the TBM cutter head
to balance the water pressure. The operators work in normal air
pressure behind the pressurised compartment, but may occasionally
have to enter that compartment to renew or repair the cutters. This
requires special precautions, such as local ground treatment or
halting the TBM at a position free from water. Despite these
difficulties, TBMs are now preferred to the older method of
tunneling in compressed air, with an air lock/decompression chamber
some way back from the TBM, which required operators to work in
high pressure and go through decompression procedures at the end of
their shifts, much like divers.

Until recently the largest TBM built was used to bore the Green
Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. It had a diameter of
14.87 m.

By special monitoring the NATM
method is very flexible, even at surprising changes of the geomechanical rock consistency during the
tunneling work. The measured rock properties lead to appropriate
tools for tunnel strengthening. In the last decades
also soft ground excavations up to
10 km became usual.

Pipe jacking

Pipe Jacking, also known as
pipejacking or pipe-jacking, is a
method of tunnel construction where hydraulic jacks are used to
push specially made pipes through the ground behind a tunnel boring
machine or shield. This technique is commonly used to create
tunnels under existing structures, such as roads or railways.

Other

Choice of tunnels vs. bridges

For water crossings, a tunnel is generally more costly to construct
than a bridge. Navigational considerations may limit the use of
high bridges or drawbridge spans
intersecting with shipping channels, necessitating a tunnel.

Bridges usually require a larger footprint on each shore than
tunnels. In areas with expensive real estate, such as
Manhattan and urban Hong Kong, this is a strong factor in tunnels' favor.Boston's
Big
Dig project replaced elevated roadways with a tunnel
system to increase traffic capacity, hide traffic, reclaim land,
redecorate, and reunite the city with the waterfront.

The 1934
Queensway Road Tunnel under the River Mersey at Liverpool, was chosen over a massively high
bridge for defence reasons. It was feared aircraft could
destroy a bridge in times of war. Maintenance costs of a massive
bridge to allow the world's largest ships navigate under was
considered higher than a tunnel. Similar conclusions were met for
the 1971 Kingsway Tunnel under the River Mersey.

Other
reasons for choosing a tunnel instead of a bridge include avoiding
difficulties with tides, weather and shipping during construction
(as in the 51.5 km Channel Tunnel), aesthetic reasons (preserving the above-ground
view, landscape, and scenery), and also for weight capacity reasons
(it may be more feasible to build a tunnel than a sufficiently
strong bridge).

There are
particular hazards with tunnels, especially from vehicle fires when
combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in
Switzerland in 2001.One of the worst railway disasters ever, the
Balvano train disaster, was
caused by a train stalling in the Armi tunnel in Italy in 1944,
killing 426 passengers.

Variant tunnel types

Double-deck tunnel

Some
tunnels are double-deck, for example the two major segments of the
San Francisco – Oakland Bay
Bridge (completed in 1936) are linked by a double-deck
tunnel, once the largest diameter tunnel in the world. At
construction this was a combination bidirectional rail and truck
pathway on the lower deck with automobiles above, now converted to
one-way road vehicle traffic on each deck.

A recent double-decker tunnel with both decks for motor vehicles is
the Fuxing Road Tunnel in Shanghai, China.
Cars travel on the two-lane upper deck and heavier vehicles on the
single-lane lower.

Multipurpose tunnel are tunnels that have more than one purpose.
The
SMART Tunnel in Malaysia is the first multipurpose tunnel in the world, as
it is used both to control traffic and flood
in Kuala
Lumpur.

Short tunnels

Artificial tunnels

Overbridges can sometimes be built by covering a road or river or
railway with brick or still arches, and then levelling the surface
with earth. In railway parlance, a surface-level track which has
been built or covered over is normally called a covered way.

Common utility ducts are
man-made tunnels created to carry two or more utility lines
underground. Through co-location of different utilities in one
tunnel, organizations are able to reduce the costs of building and
maintaining utilities.

Hazards

Owing to the enclosed space of a tunnel, fires can have very
serious effects on users. The main dangers are gas and smoke
production, with low concentrations of carbon monoxide being highly toxic. Fires
killed 11 people in the Gotthard tunnel fire of 2001
for example, all of the victims succumbing to smoke and gas
inhalation. Over 400 passengers died in the Balvano train disaster in Italy in
1944, when the locomotive halted in a long tunnel. Carbon monoxide poisoning was the
main cause of the horrifying death rate. Fires have also
occurred in the Channel
Tunnel, leading to great delays for users.

Examples of tunnels

In history

World's oldest underwater tunnel is rumored
to be the Terelek kaya tüneli under Kızıl
River, a little south of the towns of Boyabat and Duragan in Turkey.
Estimated to have been built more than 2000 years ago (possibly
5000) it is assumed to have had a defence purpose.

The
qanat or kareez of
Persia is a water
management system used to provide a reliable supply of water to
human settlements or for irrigation in hot, arid and semi-arid
climates. The oldest and largest known qanat is in the
Iranian city of Gonabad which after 2700 years still provides
drinking and agricultural water to nearly 40,000 people. Its main
well depth is more than and its length is .

The
Eupalinian
aqueduct on the island of Samos (North Aegean, Greece).Built in
520 BC by the ancient Greek engineer Eupalinos of Megara.
Eupalinos organised the work so that the tunnel was begun from both
sides of mount Kastro. The two teams advanced simultaneously and
met in the middle with excellent accuracy, something that was
extremely difficult in that time. The aqueduct was of utmost defensive
importance, since it ran underground and it was not easily found by
an enemy who could otherwise cut off the water supply to Pythagoreion, the ancient capital of Samos. The
tunnel's existence was recorded by Herodotus (as was the mole and
harbour, and the third wonder of the island, the great temple to
Hera, thought by many to be the largest in the Greek world). The
precise location of the tunnel was only re-established in the 19th
century by German archaeologists. The tunnel proper is long and
visitors can still enter it Eupalinos tunnel.

The
Via Flaminia, an important Roman road, penetrated the Furlo pass in the
Apennines through a tunnel which emperor Vespasian had ordered built in 76-77. A
modern road, the SS 3 Flaminia, still uses this tunnel, which had
an precursor dating back to the 3rd century BC; remnants of this
earlier tunnel (one of the first road tunnels) are also still
visible.

The 1796 Stoddart Tunnel in Chapel-en-le-Frith in Derbyshire is
reputed to be the oldest rail tunnel in the world. Rail wagons were
horse drawn.

The
tunnel created for the first true steam locomotive, the Penydarren locomotive, built by Richard Trevithick.The locomotive made
an historic journey from Penydarren to Abercynon in 1804.Part of this tunnel can still be seen at
Pentrebach, Merthyr Tydfil. This is arguably the oldest railway tunnel
in the world, for self-propelled steam engines on rails.

The
Montgomery
Bell Tunnel in Tennessee, a 290 foot (88 metres) long, 15 foot
wide by 8 foot high water diversion tunnel to power a water wheel,
was built by slave labor in 1819, being the first full-scale tunnel
in North America.

Crown Street Station, Liverpool, 1829. Built by George Stephenson, a single track tunnel
291 yards long was bored from Edge Hill to Crown Street to serve
the world's first passenger railway station. The station was
abandoned in 1836 being too far from Liverpool city centre, with
the area converted for freight use. Closed down in 1972, the tunnel
is disused. However it is the oldest rail tunnel running under
streets in the world. [24312]

The
1.26 mile (2.03 km) 1830 Wapping Tunnel in Liverpool, England, was the first rail tunnel
bored under a metropolis. Currently disused since 1972.
Having two tracks, the tunnel runs from Edge Hill in the east of
the city to the south end Liverpool docks being used only for
freight. The tunnel is still in excellent condition and is being
considered for reuse by Merseyrail rapid transit rail system, with
maybe an underground station cut into the tunnel. The river portal
is opposite the new Liverpool Arena being ideal for a serving
station. If reused it will be the oldest used underground rail
tunnel in the world and oldest part of any underground metro
system.

1836, Lime St Station tunnel, Liverpool. A two track rail
tunnel, 1.13 miles (1,811 m) long was bored under a metropolis from
Edge Hill in the east of the city to Lime Street. In the 1880s the
tunnel was converted to a deep cutting four tracks wide. The only
occurrence of a tunnel being removed. A very short section of the
original tunnel still exists at Edge Hill station making this the
oldest rail tunnel in the world still in use, and the oldest in use
under a street, albeit only one street and one building.

Box
Tunnel in England, which opened in 1841, was the longest
railway tunnel in the world at the time of construction. It
was dug and has a length of .

The 0.75 mile long 1842 Shildon tunnel near Darlington,
England, is the oldest sizable tunnel in the world still in use
under a settlement.

The
2.07 miles (3.34 km) Waterloo Tunnel in Liverpool, opened in 1849, was bored under a
metropolis. Initially used only for rail freight and later
freight and passengers serving the Liverpool ship liner terminal,
the tunnel runs from Edge Hill in the east of the city to the north
end Liverpool docks. Used until 1972 it is still in excellent
condition, being considered for reuse by the Merseyrail rapid transit rail system. Stations
being cut into the tunnel are being considered. Also, reuse by a
monorail system from the proposed Liverpool Waters redevelopment of Liverpool's Central Docks has been
proposed.

The oldest sections of the London
Underground were built using the cut-and-cover method in the
1860s. The Metropolitan, Hammersmith & City, Circle and
District lines were the first to prove the success of a metro or subway system. Dating from 1863,
Baker Street
station is the oldest underground station in the
world.

The
1882 Col de Tende
Road Tunnel, at 3182 metres long, was one of the first long
road tunnels under a pass, running between France and
Italy.

The Mersey Railway tunnel opened
in 1886 running from Liverpool to Birkenhead under the River
Mersey. By 1892 the extensions on land from Birkenhead Park station
to Liverpool Central Low level station gave a tunnel 3.12 miles
(5029 m) in length. The under river section is 0.75 miles in
length, being the longest underwater tunnel in world in January
1886.

The rail Severn Tunnel was opened
in late 1886, at 4 miles 624 yd (7,008 m) long, although only 2¼
miles (3.62 km) of the tunnel is actually under the river. The
tunnel replaced the Mersey Railway tunnel's longest under water
record, which it held for less than a year.

James Greathead, in constructing
the City & South London Railway tunnel beneath the Thames,
opened in 1890, brought together three key elements of tunnel
construction under water: 1) shield method of excavation; 2)
permanent cast iron tunnel lining; 3) construction in a compressed
air environment to inhibit water flowing through soft ground
material into the tunnel heading.

St. Clair Tunnel, also opened later in 1890, linked the elements of
the Greathead tunnels on a larger scale.

Gerrards Cross tunnel in Britain is notable in that it is being built
over a railway cutting that was dug in the early part of the 20th
Century. Thus, arguably, making it the tunnel longest in
construction by the cut and cover method. When complete a
branch of the Tesco
supermarket chain will occupy the space above the
railway.

The
La Linea Tunnel in
Colombia, is the longest mountain tunnel in South
America. It crosses beneath a mountain at above sea level
with six lanes and it has a parallel emergency tunnel. The tunnel
is subject to serious groundwater pressure. The tunnel, which is
currently under construction, will link Bogota and its
urban area with the coffee-growing region and with the main port on
the Colombian Pacific coast.

Multiple headings

Long tunnels get increasingly slow to build as they get longer. A
solution if the geography suits is to have intermediate access
shaft so that excavation can take place on several faces. Surveyors
have a good record of making the separate sections meet with errors
in alignment of just a few inches. An exception would be one of the
hundred tunnels on the Kalka-Shimla
Railway.

Other uses

Excavation techniques, as well as the construction of underground
bunkers and other habitable areas, are often associated with
military use during armed conflict,
or civilian responses to threat of attack. The use of tunnels for
mining is called drift mining. One of
the strangest uses of a tunnel was for the storage of chemical
weapons [24314].

Natural tunnel

Natural Tunnel State Park (Virginia, USA) features an natural tunnel,
really a limestone cave, that has been used as a railroad tunnel
since 1890.

Punarjani Guha Kerala, India.
Hindus believe that crawling through the tunnel (which they believe
was created by a Hindu god) from one end to the other will wash
away all of one’s sins and thus attain rebirth, although only men
are permitted to crawl through the cave.

Snow tunnels are created by voles, chipmunks and
other rodents for protection and access to food sources. Larger
versions are created by humans, usually for fun.

Temporary Way

During construction of a tunnel it is often convenient to install a
temporary railway particularly to remove spoil. This temporary
railway is often narrow gauge so that it can be double track, which
facilitates the operation of empty and loaded trains at the same
time. The temporary way is replaced by the permanent way at completion, thus explaining
the term Perway.

The 1836 Lime Street two track 1 mile tunnel from Edge Hill to Lime
Street in Liverpool was totally removed, apart from a short 50
metre section at Edge Hill. Four tracks were required. The tunnel
was converted into a very deep 4 track open cutting. However, short
larger 4 track tunnels were left in some parts of the run. Train
services were not interrupted as the work progressed. Photos of the
work in progress: [24315][24316]There are other occurrences of tunnels being
replaced by open cuts, for example, the Auburn Tunnel.